1. Field of the Invention
This invention relates generally to water-based, solids-containing fluids and more particularly concerns drilling fluids and drilling fluid additives that inhibit the hydration of colloidal clays, such as shales, gumbos and related geologic formations.
2. Description of the Prior Art
In recent years, the importance of drilling for oil, gas and other valuable earth-bound formations has increased. Unfortunately, such drilling is an unpredictable, expensive and often frustrating endeavor.
In the formation of a subterranean well, a drilling fluid is circulated to the bottom of a borehole and ejected into the borehole from a drill bit. The drilling fluid usually returns to the surface of the well through the annulus of the borehole. Alternatively, the drilling fluid rises to the surface from the bottom of the borehole through a drill stem. At the surface, the drilling fluid is chemically and mechanically treated to provide desired properties for recycling. Additional fluid and additives can be incorporated at the surface to enhance performance.
To perform the boring operation effectively, the drilling fluid must possess a variety of properties. For example, the drilling fluid should be capable of (1) transporting the cuttings, resulting from the drilling operation, out of the borehole; (2) cooling and lubricating the drill bit; (3) providing hydrostatic pressure to the borehole to prevent the hole from caving in or blowing out while using drilling fluids in a high pressure environment; (4) providing a wall cake upon the borehole wall; (5) avoiding excessive amounts of fluid loss during drilling operations; and (6) suspending the solid particles from the drilling operation when the drilling fluid ceases to circulate in the borehole.
As is well-known, aqueous-based drilling fluids are usually comprised of water and one or more additives or dispersants such as colloidal solids, non-colloidal solids, chemical dispersants, thinners, and dissolved salts. Water alone has been used in drilling short distances, but it lacks the lubricity, lifting power and hole-supporting properties provided by conventional deep well additives. Hence, solids-containing fluids with greater density, lifting power, and hole-supporting properties than water are often employed as drilling fluids. The properties of drilling fluid may be affected by the types of rocks through which the subterranean well is drilled. Usually, the viscosity, temperature, and pressure of the fluid are altered as the borehole is drilled through a rock formation.
Various additives and dispersants have been proposed that provide the requisite properties in drilling fluids. For example, bentonite, lignite, phosphate, and lignosulfonate-containing materials are often added.
Bentonite gives the fluid a thixotropic or gel structure. Lignite is added to the drilling fluid to control the thixotropy of the drilling fluid. Phosphate additives deflocculate colloidal solids and thin the drilling fluid. Like lignite, phosphate additives lose their effectiveness as temperature and pressure of the drilling fluid increase. Lignosulfonates are also used as a drilling fluid additive and exhibit good deflocculating properties when used in calcium or salt-containing drilling fluids.
In drilling, it is often softer rock formations which present the greatest problems, especially colloidal clays, such as shales and gumbos, which have a tendency to hydrate and swell when exposed to aqueous solutions. This swelling results in the formation of gummy, sludge-like paste in the drill hole which reduces the cutting efficiency of the drill bit, thickens and impedes the circulation of drilling fluid, plugs circulation and otherwise delays progress.
When the circulation of drilling fluid is impeded, a substantial amount of cuttings can build up in the hole and exert pressure against the rotating drill string. Hence, more torque is required to maintain the rotary motion of the drill. This is particularly true in drilling shales and related rock types where not only does the volume of the cuttings increase when hydrated, but the absorption of water from the drilling fluid and dispersion of the shale increase both the viscosity of the circulating fluid and the torque demand on the drilling unit. Occasionally, enough pressure can be exerted to cause the string to break or separate, and the resultant down time can cost tens of thousands of dollars per day.
Another problem is that shale and gumbo cuttings can stick to the surface of the drill bit and reduce its cutting efficiency by forming a layer between the bit and the formation being drilled. This phenomenon, sometimes called "balling up," results in a substantial decrease in drill penetration rate and adds to overall costs.
Still another problem associated with drilling these formations is the "cleanness" or "gauge" of the drill hole. Often, the walls of the hole collapse or become coated with hydrated cuttings. This makes it difficult to analyze the geologic structures penetrated.
Conventionally, an "electro log," including a radioactive source and detection mechanism, (or some other means of measuring the characteristics of the formations being drilled), is lowered down the hole and a chart is used to record data on the rock formations over the length of the hole. While drilling colloidal clays, however, the sludge of hydrated cuttings may stop the progress of the logging device, or impede its operation.
To alleviate the difficulties associated with drilling different types of geologic formations, various drilling fluid additives have been proposed that decrease friction, improve fluid circulation, seal up empty pockets in the earth, prevent cave in, decrease water loss and exhibit hydration. In addition to some of the additives mentioned above, chemical thinners and water loss control agents have been added to drilling fluid to increase flow and improve circulation. Lubricants, plastic particulates, and fibrous additives have also been used to reduce friction or shore up the walls of the drill hole to prevent collapse.
It has previously been discovered that exposing colloidal clays such as shales, gumbos and like formations to substantial concentrations of polyvalent metal ions can inhibit swelling and absorption of water. Generally, the polyvalent metal ions are complexed with a chelating agent prior to adding them to the drilling fluid. The ions are generally more resistant to fluctuations in pressure and temperature than are many of the agents previously discussed.
Unfortunately, a number of the polyvalent metal complexes currently used to inhibit hydration are not as effective as desired. Many also require the addition of large amounts of caustic materials because the acidic character of the complexes creates a danger that persons handling them will be chemically burned and because it is often desirable for drilling fluids to have an alkaline pH. The danger of burns is especially high in the close-quartered environment of an off-shore drilling platform where, due to the confined nature of the work place, the likelihood of coming into contact with materials present on the rig is high.
One type of polyvalent metal colloidal clay-stabilizing additive is described in Chesser et al., (U.S. Pat. No. 3,762,485), directed to a process for inhibiting the swelling of shale, including: (1) forming polyvalent metal ion complexes with agents selected from the class consisting of acetic, citric, formic, lactic, oxalic, and tartaric acids and metal and ammonium salts thereof and (2) adjusting to a pH above 7.0.
However, it is still desired to further reduce hydration in colloidal clays and to avoid adding relatively large amounts of sodium or potassium hydroxide or other caustics to neutralize the complex and to achieve an alkaline pH. As might be expected, each unit of hydroxide or other neutralizing agent increases both supply costs of the drilling operation and the need for transportation and storage space.
Generally, it is desirable to adjust the pH of the drilling fluid to an alkaline value during drilling operations, and caustic soda, potash or related alkali metal hydroxides are used. The caustics are normally added to the drilling fluid at the surface of the well. The presence of acidic agents increases the amount of caustic material necessary to achieve a high pH.
Hence, a drilling fluid composition which more effectively inhibits hydration in colloidal clays and which requires smaller amounts of caustic materials is desirable.